You can
Beginning a stitching project with two colors, is easy. You would follow the same procedure as if you had been sewing, and your thread had broken. To neatly Join two threads in an ordinary knot, now to start sewing, you would make your first stitch in what ever hole you wish, but leave the knot on top of the leather above outside on the thin edge your first stitch, don't try and burry it. Make the first stitch, then begin sewing as usual. You might even do a couple of back stitches and proceed forward. Then you can clip/snip the knot off. Seamless...
Just as with a single thread, we line up our two needles with the thread in the center of the hole, but here, we begin with the knot on top of the center then push both needles through the first hole, leaving our first stitch around the vertical of the first hole, then proceed stitching horizontal as usual.
It's a smart method of joining two shorter pieces of waxed thread that were left over from a previous project.
That knot, between two joined strands, has been a concern of many leather workers and other craftsman.
This solution, seems one of the simplest, while saving the integrity of your previous stitches.
I don't see why four colors couldn't be used. ;-)
Chas